1. Field of the Invention
The present invention relates generally to communication systems, and in particular, to a method and apparatus for performing a handoff.
2. Description of the Related Art
In a mobile communication environment, as a user moves from the coverage area of one base station to the coverage area of another base station, a handoff must occur to transition the communication link from one base station to the next. Handoff determinations are typically made based on signal strength measurements by mobile stations of pilot signals transmitted by respective base stations. If the measured pilot signal strength of the present base station falls below a threshold, the mobile station transmits a pilot strength measurement message (PSMM) which is forwarded to a transceiver and selector bank (TSB) of a base station controller (BSC). The base station controller then determines what type of handoff should be performed.
Handoffs are generally classified into two types. The first type is known as a soft handoff. For a soft handoff, a mobile station MS simultaneously maintains connection with two or more base stations (i.e. make before break). That is, as the mobile moves from its current cell (source cell) to the next cell (target cell), a traffic channel is simultaneously maintained with both cells. A soft handoff usually occurs when a mobile station travels from one cell to another cell served by the same BSC, where the base station of the second cell uses the same frequency assignment as the first.
The second type of handoff, hard handoff, is an abrupt handoff in which the mobile station is not controlled simultaneously by two or more base stations. Unlike the soft handoff, the call link connected to the mobile station is not continuously maintained, it is instead, cut-off from a base station located in a source cell and then re-established in a very short time frame with a base station from a target cell (i.e. break before make).
A conventional handoff in a mobile communication system will be described with reference to FIG. 1.
Before describing a conventional handoff procedure, terms used herein will be defined.
Mode 0 (BS transmission mode 0 and MS reception mode 0): A BS normally transmits data for the entire frame period and an MS receives the data.
Mode 1 (BS transmission mode 1 and MS reception mode 1): The BS transmits data for a part of the frame period and the MS received the data.
Mode 2 (BS transmission mode 2 and MS reception mode 2): During part of the frame period where the BS is not transmitting data, the MS searches for an adjacent BS.
A guard time required to transit from mode 1 to mode 2 is called a and a guard time required to transit from mode 2 to mode 1 or mode 0 is called b.
First frame: The first frame transmitted to the MS by the BS upon request for a handoff.
Second frame: A frame following the first frame.
Slotted Mode (Compressed Mode): An operation mode of the BS in which the BS divides a frame period into time slots and transmits data only in selected slots. A data transmission period is called an action period in the slotted mode and a non-data transmission period is called a non-action period in the slotted mode.
FIGS. 1A and 1B illustratively depict a conventional handoff in slotted mode 1 and in slotted mode 2, respectively.
Referring to FIG. 1A, a BS communicates with an MS in mode 0 in step 100. Mode 0 is a transmission scheme in which data at a transmission rate RD is spread by a layer-m orthogonal code and transmitted for a frame period T. Upon require for a handoff, the BS doubles the data transmission rate, spreads data by a layer-(mxe2x88x921) orthogonal code for the first half of the frame period for transmission, and transmits no data for the last half of the frame period, in steps 110 and 120. Therefore, the MS receives the data from the BS for the first half of the frame period at the doubled data transmission rate and searches for an adjacent target BS to which a handoff will occur for the last half frame period. Then at steps 130 and 140, the BS transmits data spread by the layer-(mxe2x88x921) orthogonal code at the doubled data transmission rate for the first half frame period and then transmits no data for the last half frame period. Once again, the MS receives the data from the BS for the first half frame period and then searches for the adjacent BS to which a handoff will occur for the last half frame period.
As stated above, upon require for a handoff, the BS transmits data for the first half of the first and second frame period, and the BS does not transmit any data to the MS in the last half of the first and second frame periods, to allow the MS to search for an adjacent BS, in slotted mode.
Now referring to FIG. 1B, the BS communicates with the MS in mode 0 in step 200. Mode 0 is a transmission scheme in which data at the transmission rate RD is spread by the layer-m orthogonal code and transmitted for the frame period T. Upon require for a handoff, the BS doubles the data transmission rate, spreads data by the layer-(mxe2x88x921) orthogonal code for the first half of the first frame period for transmission in step 210, and transmits no data for the last half of the first frame period 220. T
Therefore, the MS receives the data from the BS for the first half of the first frame period 210 and searches for an adjacent target BS in last half of the first frame and first half of the second frame period 220. Then, in steps 220 and 230, the BS transmits no data for the first half of second frame period and transmits data spread by the layer-(mxe2x80x941) orthogonal code at the doubled data transmission rate for the last half of second frame period 230. That is, upon request for a handoff, the BS transmits data for the first half of the first frame period and the last half of the second frame period, and the MS searches for the adjacent BS in the last half of the first frame period and the first half of the second frame period without receiving data, in slotted mode 2.
FIG. 2 illustrates orthogonal code layers which have variable spread gains and maintain orthogonality among channels.
Referring to FIG. 2, orthogonal codes in the same layer are mutually orthogonal and orthogonal codes in a direct line are not orthogonal. Therefore, either a direct upper layer (m+k) (k=0, 1, 2, . . . ) orthogonal code or a direct lower layer (mxe2x88x92k) (k 0, 1, 2, . . . , m) orthogonal code cannot maintain orthogonality among channels with respect to a layer-m (m=0, 1, 2, . . . ) orthogonal code.
FIGS. 3A and 3B illustrate orthogonal code layers to describe an upper layer orthogonal code assigning method when a conventional handoff between frequencies is to be implemented. In the drawings, orthogonal codes marked with rectangles (in layer 3) represent the current handoff candidate (i.e., requiring a handoff) and orthogonal codes marked with oval circles have assigned to channels in current communication.
Referring to FIGS. 3A and 3B, it is assumed that while the BS transmits using an orthogonal code 00000000, a handoff occurs. If an orthogonal code 0000 in the direct upper layer is available as shown in FIG. 3A, the BS transmits data with use of 0000. However, if the orthogonal code 0000 cannot be assigned due to an orthogonal code 00001111 in current use as shown in FIG. 3B, the BS detects an orthogonal code available among other orthogonal codes in the direct upper layer. Recall that orthogonal code 0000 in FIG. 3B cannot be assigned because it is in a direct line with 00001111, which it is not orthogonal each other. Then, the BS determines that orthogonal code 0011 is available and is not in a direct line with code 00001111 and transmits data with use of the orthogonal code 0011. In this case, different orthogonal codes may be used in steps 100 and 150 of FIG. 1A and in steps 200 and 240 of FIG. 1B. The probability of using a different orthogonal code from an orthogonal code in a previous period is higher in FIG. 3B than in FIG. 3A because the original orthogonal code can be used all the time due to use of the orthogonal code in the direct upper layer in the case shown in FIG. 3A. On the other hand, if the original orthogonal code is assigned to another channel in the slotted mode, it is impossible to return the original orthogonal code in the case of FIG. 3B.
A conventional hard handoff procedure will be described referring to FIG. 4.
A current BS (BS0) transmits data for the entire frame period T in step 411 and a mobile station (MS) receives the data in step 413. The BS continues communication with the MS in step 415. The MS measures the strength of a signal received from the BS in step 417, and notifies the BS of the measurement if the signal strength is at the threshold level or below in step 419. Then, the BS determines whether a handoff is required based on the measurement in step 421. Upon require for a handoff, the BS goes to step 423. Otherwise, if the BS determines that no handoff has been requested, the BS returns to step 411 in which it transmits data for a time T again. In step 423, the BS determines whether there is an available orthogonal code in a direct upper layer. If an orthogonal code currently in use is in layer 2, the BS determines whether there is A any available orthogonal code in layer 1. In the presence of an available orthogonal code, the BS goes to step 425, and otherwise, it awaits generation of an available orthogonal code in the direct upper layer in step 423. That is, the handoff cannot be performed until an available orthogonal code is generated. In step 425, the BS transmits to the MS various parameters (e.g., orthogonal code, transmission period and non-transmission period) required for the handoff. Then, the MS receives the handoff-related information in step 427 and notifies the BS of the reception status in an acknowledgment signal ACK in step 429. The BS determines whether the acknowledgment signal ACK has been received from the MS in step 431. Upon reception of the signal ACK, the BS goes to step 433, and otherwise, it returns to step 425 to resume transmission of the handoff-related information. In step 433, the BS spreads data at a doubled transmission rate, T/Ton, where Ton is T/2 and thus T/Ton is 2, using the orthogonal code in the upper layer and transmits the spread data for a time Ton in the first half period D1 of a first frame. Then, the MS receives the spread data in the first half period D1 and a signal from an adjacent BS for the last half period D2 of the first frame to thereby search for a new BS for the handoff in step 435.
Referring to FIG. 5, a problem with the conventional handoff procedure described above is that an orthogonal code in the upper layer cannot be assigned in the case where a channel using an orthogonal code 00000000 in layer 3 temporarily increases its data transmission rate to implement a handoff. For example, the orthogonal code 0000 in layer 2 is not available due to an occupied orthogonal code 00001111 in layer 3. Similarly, an orthogonal code 0011 in layer 2 is not available due to an occupied orthogonal code 00110011 in layer 3. An orthogonal code 0101 in layer 2 is in current use and an orthogonal code 0110 in layer 2 cannot be assigned due to an occupied orthogonal code 01101001 in layer 3. That is, no orthogonal codes in layer 2 are available to maintain orthogonality. Therefore, the handoff cannot be performed until an available orthogonal code in layer 2 is generated. This problem can be overcome by separately reserving orthogonal codes in the upper layer for the handoff. However, the orthogonal code reservation decreases channel use efficiency when the frequency of handoff occurrences is low, and is inefficient in a system supporting a variable data rate since different orthogonal codes are needed at different data rates.
FIG. 6 illustrates another conventional handoff implementing method. Referring to FIG. 6, if the orthogonal code 00001111 in layer 3 is assigned to a channel in current use the orthogonal code 0000 in the direct upper layer (i.e., layer 2) is not available, then either orthogonal code 00111100 or 01100110 available in the same layer will then be assigned for the channel that uses orthogonal code 00001111. That is, the orthogonal code 00001111 is returned and the orthogonal code 0000 is assigned to a channel for the handoff. To ensure a reliable handoff, a control signal should be used between the BS and the MS in this method. If a handoff is implemented with the orthogonal code 00110011 during a call in progress with the newly assigned orthogonal code 00111100, the above procedure should be performed again.
It is, therefore, an object of the present invention to provide a hard handoff implementing device and method in a mobile communication system, in which a frame period is divided into a transmission period and a non-transmission period and frame data is spread with a multicode for transmission in the transmission period when the upper layer orthogonal code cannot be assigned.
The above object is achieved by a handoff implementing device and method in a mobile communication system. The handoff implementing device includes a base station transmitter and a mobile station receiver. The base station transmitter divides a given frame period into a transmission period and a non-transmission period, separates frame data in the frame period into first and second data, spreads the first and second data by different orthogonal codes, and transmits both spread signals in the transmission period. The mobile station receiver receives the first and second data spread by the different orthogonal codes in the transmission period, assembles the first and second data into the frame data, and searches for an adjacent base station to which a call is handed off in the non-transmission period.